1. Field of the Invention
The present invention relates generally to lithographic apparatus and more particularly to lithographic apparatus including spatially modulated illumination nodes.
2. Description of the Prior Art
The term “patterning device” or “patterning structure” as here employed should be broadly interpreted as referring to devices that can be used to endow an incoming The present invention relates generally to lithographic apparatus and more particularly to lithographic projection apparatus having image correcting functionality. The term “patterning device” or “patterning structure” as here employed should be broadly interpreted as referring to devices that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning device include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmission mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, said support structure for holding a patterning device will also be referred to hereinafter as “mask table”; the mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and it can be moved relative to the beam if so desired;        A programmable mirror array. One example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as non-diffracted light. Using an appropriate filter, the said non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localised electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic controllers. In both of the situations described here above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required. For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning devices as set forth here above.        
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.
The mask as introduced in the present text is also known as a “reticle”, which term may be used hereinafter. Also, the mask table as described above may be referred to hereinafter as the “reticle stage”. The position of the reticle stage and the substrate table are generally measured with one or more laser interferometers to obtain the necessary accuracy for positioning of the substrate during exposure and for mutual alignment of successive patterned layers of a device.
A reticle can be protected by a soft and/or thin pellicle to prevent damage to the pattern plane on the reticle. Such a soft pellicle typically comprises a thin polymer foil which is substantially parallel to the surface of the reticle, displaced at some substantially perpendicular distance to the reticle's surface.
For example, particles on the reticle could disadvantageously affect the pattern image generated by radiation impinging on the reticle. By intercepting particles on the pellicle on a surface which is not in the focal plane of the projection system, the quality of the image generated from the (clean) reticle can be substantially preserved.
It has been observed that short-wavelength radiation, such as deep ultraviolet radiation (e.g., 126 and 157 nm) and extreme ultraviolet radiation (EUV) (e.g., 5–20 nm) deteriorate the soft pellicle so rapidly that the use of a soft pellicle in combination with (deep) UV (or EUV) radiation may not be practical at this moment.
A replacement pellicle is being considered for reticles used in (deep) UV/EUV lithography applications. This replacement pellicle can withstand the (deep) UV/EUV radiation and comprises a thin glass plate which is transparent for the (deep) UV/EUV radiation. Due to the refracting properties of a glass plate (based on Snell's law), the radiation beam is refracted in such a way that an offset of the focal plane of a projection system for the generated pattern image occurs, and the location of the reticle relative to the projection system is changed accordingly.
However, such a change of the reticle location adversely affects the ability to use reticles without a pellicle during some procedures. For example, in wafer fabs for IC production many lithography tests are performed (some even on a routinely basis) which require the use of a pellicle-free reticle. A reticle without a pellicle may be used in such circumstances because the pattern may be more accurate without a pellicle.
The switching between reticles with or without a pellicle requires cumbersome and time-consuming modification of a lithographic projection apparatus and a reduction in the operational up-time of the apparatus. Preferably, an exchange of reticles with and without a pellicle should be performed within a few minutes (e.g., less than 10 min).